notifications-page with HTML, CSS and Javascript

Today I finished my notifications-page. I have doubts about three things:

• It was difficult to align the elements in my header element. Now I used flex box and gave the p element a width of 90%. Is there a better solution?

• I had problems recreating the design in the right size. The font-size looked to big on a 375px device to look exactly like the design. After I changed it to a 425px device everything fitted in to place.

• I made the notifications toggle to read individually ass well as the 'mark all ass read link'. Was it supposed to let it function only on the 'mark all ass read' link?

If you have any constructive criticism please feel free to let me know.